Bad news. I've had some spare time to take a deeper look at this, but I still
haven't found a way to solve it.
Using an absolutely dirty hack I'm able to get filemerge to use the correct
ancestor:
merge against 2:7e1afe9214b2
resolving manifests
 ancestor 0a6620c3c26a local 0b69665d48b4+ remote 7e1afe9214b2
picked tool 'internal:merge' for a (binary False symlink False)
my a@0b69665d48b4+ other a@7e1afe9214b2 ancestor a@0a6620c3c26a

but again I get:
warning: conflicts during merge.
merging a failed!
abort: fix unresolved conflicts with hg resolve then run hg rebase --continue

Starting from the next week I'll be very busy again. I'm afraid I won't work on
this before 1.3.

As a future reference: mpm said to take a peek at issue1327
I'm adding mpm to the nosy list...